Wide-band acoustically coupled thin-film baw filter

ABSTRACT

The invention relates to an acoustically coupled thin-film BAW filter, comprising a piezoelectric layer, an input-port on the piezoelectric layer changing electrical signal into an acoustic wave (SAW, BAW), and an output-port on the piezoelectric layer changing acoustic signal into electrical signal. In accordance with the invention the ports include electrodes positioned close to each other, and the filter is designed to operate in first order thickness-extensional TE1 mode.

FIELD OF INVENTION

The present invention relates to a wide-band acoustically coupledthin-film Bulk Acoustic Wave (BAW) filter according to the preamble ofClaim 1.

BACKGROUND OF THE INVENTION

Radio-frequency (RF) components, such as resonators and filters, basedon microacoustics and thin-film technology are widely used in radioapplications such as: mobile phones, wireless networks, satellitepositioning, etc. Their advantages over their lumped-elementcounterparts include small size and mass-production capability. Twofundamental microacoustic technologies used for RF devices are surfaceacoustic wave (SAW) and bulk acoustic wave (BAW) technologies.

In this section, existing filter technologies are briefly introduced toprovide background for the current invention.

Surface Acoustic Wave Devices

A schematic picture of a SAW device is shown in FIG. 1(a). Interdigitaltransducers (IDTs) 2, 3 (comb-like structures of thin-film metal strips)are patterned on a piezoelectric substrate 1. The piezoelectricsubstrate is, for example, quartz, LiNbO3 or LiTaO3. The IDTs are usedto transform the electric input signal Vin into an acoustic wave via thepiezoelectric effect, as well as to pick up the acoustic signal at theoutput port and transform it back to an electrical form. The operationfrequency of a SAW device depends on the velocity of the acoustic waveand the dimensions of the IDT electrodes:

$f \propto \frac{v}{2\; p}$

where f is the frequency, p is the period of the IDT, see FIG. 1(b), andv is the velocity of the surface wave. Therefore, higher operationfrequencies require smaller p if the velocity is kept constant.

SAW transducers are typically periodic, although the period may be morecomplex than that presented in FIG. 1.

Bulk Acoustic Wave Devices

In a BAW device, acoustic vibration inside a piezoelectric wafer or athin film is used to process the electrical input signal. Thepiezoelectric material used for the thin film in the devices typicallybelongs to the 6 mm symmetry group, e.g. ZnO, AlN and CdS. Otherpiezoelectric materials can be used as well, such as quartz, LiNbO₃,LiTaO₃, etc. A schematic cross-section of a solidly-mounted BAWresonator (SMR) is presented in FIG. 2(a) and of a self-supported(membrane type) resonator in FIG. 2(b). In an SMR, an acoustic Braggmirror composed of alternating high and low acoustic impedance (Z)material layers serves to isolate the vibration in the piezoelectricthin film from the substrate and to prevent acoustic leakage. In themembrane device the same is accomplished by fabricating the resonator ona self-standing membrane.

The area of a BAW resonator is typically determined by the staticcapacitance needed to match the device to the system impedance. Filterscan be constructed of resonators by connecting resonators electrically.A common example is a ladder filter, in which resonators are connectedin T- or Pi-sections (FIG. 3). Designing the resonance frequenciesappropriately, one can achieve a passband response. Increasing thenumber of sections helps to widen the passband. The out-of-band signalsuppression is determined by the capacitances of the resonator structureand is typically on the order of ˜25 dB. The in-band losses are mainlydetermined by the Q-values of the resonators.

Vibration Modes and Dispersion Types

In the piezoelectric layer of an acoustic resonator, different bulkacoustic vibration modes arise as the excitation frequency f is swept.In BAW devices, the propagation direction of the bulk wave is typicallyalong the thickness axis (z axis). Particle displacement is eitherperpendicular to the propagation direction (shear wave) or parallel tothe propagation direction (longitudinal wave). Bulk modes arecharacterized by the number of half-wavelengths of the bulk wavelengthλ_(z) that can fit into the thickness of the resonator structure(piezoelectric layer and electrodes). In addition, the bulk modes canpropagate in the lateral (perpendicular to z-axis) direction as platewaves with lateral wavelength λ₈₁. This is illustrated in FIG. 4a fortwo bulk modes (longitudinal and shear). In a finite-sized resonator,plate waves reflecting from resonator edges can cause laterally standingwaves and consequently lateral resonance modes.

Acoustic properties of a BAW resonator can be described with dispersioncurves, in which the lateral wave number k_(∥) of the vibration ispresented as a function of frequency. FIG. 4b shows an example ofdispersion properties in a BAW resonator. Dispersion curves of theelectroded (active) region are plotted with a solid line and those ofthe non-electroded (outside) region with dashed line. The first-orderlongitudinal (thickness extensional, TE1) vibration mode, in which thethickness of the piezoelectric layer contains approximately half awavelength of the bulk vibration, and the second-order thickness shear(TS2) mode, in which the bulk vibration is perpendicular to thethickness direction and one acoustic wavelength is contained in thepiezoelectric layer thickness, are denoted in the figure. This type ofdispersion, in which the TE1 mode has increasing k_(∥) with increasingfrequency, is called Type 1. Type 1 materials include, e.g. ZnO.Aluminum nitride is inherently Type 2 (in FIG. 4b , TE1 mode is thelower dispersion curve, and TS2 mode is the upper dispersion curve), butwith an appropriate design of the acoustic Bragg reflector, theresonator structure's dispersion can be tailored to be of Type 1.

In FIG. 4b , positive values of k_(∥) denote real wave number(propagating wave) and negative values correspond to imaginary wavenumber (evanescent wave). For a resonance to arise, the acoustic energymust be trapped inside the resonator structure. In the thicknessdirection, isolation from the substrate (mirror or air gap) ensures theenergy trapping. In the lateral direction, there should be an evanescentwave outside the resonator region for energy trapping. Energy trappingis possible between frequencies f_(o1) and f_(o2). Frequency range forwhich energy trapping occurs for the TE1 mode, f_(o2)-f_(a), is shadedin FIG. 4b . Energy trapping is easier to realize in Type 1 dispersion.Therefore, when using AlN as the piezoelectric material, the mirror isusually designed so that it converts the dispersion into Type 1. Thislimits somewhat the design of the acoustic mirror.

As the lateral wave number k_(∥) increases on a dispersion curve(lateral wavelength decreases), lateral standing wave resonances (platemodes) appear in the resonator structure. For a plate mode to arise, thewidth of the resonator W must equal an integer number of halfwavelengths of the plate mode:

$W = {N\; \frac{\lambda_{m}}{2}}$

for the mode m with wavenumber k_(m)=2π/λ_(m).

Acoustical Coupling in BAW Devices

A filter can be made by electrically connecting one-port resonators toform a ladder or a lattice filter. Another possibility is to arrangemechanical (acoustic) coupling between resonators by placing them closeenough to each other for the acoustic wave to couple from one resonatorto another. Such devices are called coupled resonator filters (CRF).

In BAW devices, vertical acoustic coupling between stacked piezoelectriclayers is used in stacked crystal filters (SCF, see FIG. 5(a)) andvertically coupled CRFs (FIG. 5 (b)). In an SCF, two piezoelectriclayers are separated by an intermediate electrode. In a verticallycoupled CRF, coupling layers are used to modify the coupling strengthbetween the piezo layers. The CRF can be fabricated either using the SMRor the air-gap technology.

A thin-film vertically coupled CRF has been shown to give a relativelywide-band frequency response (80 MHz at 1850 MHz center frequency, or4.3% of center frequency, FIG. 8(a) from G. G. Fattinger, J. Kaitila, R.Aigner and W. Nessler, “Single-to-balanced Filters for Mobile Phonesusing coupled Resonator BAW Technology”, Proc. IEEE UltrasonicsSymposium, 2004, pp. 416-419) with the capability ofunbalanced-to-balanced (balun) conversion. The disadvantage of thevertically coupled CRFs is the need for a large number of layers andtheir sensitivity to the thickness of the piezolayers. This makes thefabrication process difficult and consequently expensive.

Lateral acoustical coupling in BAW (LBAW) can be realized with 2 or morenarrow electrodes placed on the piezoelectric layer 1 (FIG. 6) on a thinlayer structure 4, in such a way that the acoustic vibration can couplein the lateral direction from one electrode to another. Electrical inputsignal in Port 1, 5 is transformed into mechanical vibration via thepiezoelectric effect. This vibration couples mechanically across the gapto Port 2, 6 and creates an output electrical signal. Electrodes in thisexample are interdigital (comb-like). Coupling strength is determined bythe acoustic properties of the structure and by the gap between theelectrodes.

Bandpass frequency response is formed by two laterally standingresonance modes arising in the LBAW structure, as illustrated in FIG. 7for a two-electrode structure. In the even mode resonance, bothelectrodes vibrate in-phase, whereas in the odd mode resonance theirphases are opposite. For a Type 1 resonator, the even mode, having alonger wavelength, is lower in the frequency than the shorter-wavelengthodd mode. The frequency difference between the modes determines theachievable bandwidth of the filter, and depends on the acousticproperties of the structure and on the electrode dimensions.

Vertical CRF's have disadvantages as they are difficult and costly tofabricate, i.e. they require several layers and are sensitive topiezoelectric films' thickness.

LBAW is therefore advantageous because it has a simple fabricationprocess and the operation frequency is mainly determined by thepiezolayer thickness, though to a lesser extent by electrode geometry.So far however, the obtained bandwidth has been too narrow, i.e. 2%-3%of the center frequency.

One problem with LBAW filters is the gently sloping edges of the passband, which can limit the area of application of the components. Bysteepening the sloping edges, the competitiveness of LBAW filters wouldimprove significantly.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a wide-bandacoustically coupled bulk acoustic wave (BAW) filter based on thin-filmtechnology and piezoelectric thin films.

An aspect of embodiments of the present invention is that the filtersare operable in the GHz frequency range.

More specifically, the filter according to the invention ischaracterized by what is stated in the characterizing portion of Claim1.

Considerable advantages are gained with the aid of the invention.Examples of advantages which can be obtained through embodiments of theinvention are as follows.

Wide frequency bandwidth at GHz frequencies is possible. At least insome embodiments, the frequency bandwidth is better than that of knownacoustic wave components.

The invention also has embodiments, in which there is the possibility tooperate the filter at high frequencies, e.g. 1-5 GHz. As frequency isdetermined mainly by film thicknesses, this means that lithography isnot necessarily a principal limiting factor, as it is for SAWcomponents.

Further advantages achievable by means of at least some embodiments arethe relatively simple fabrication process, e.g. established BAW processwith only one piezoelectric layer, compared to competingcoupled-resonator BAW technologies, comparatively small component sizecompared to, e.g. SAW and ladder BAW components, and high out-of-bandsuppression compared to, e.g. ladder BAW filters.

It is a further objective of one or more embodiments of the presentinvention to offer a solution relating to the steepening of thestop-band attenuation of LBAW filters.

Also, some embodiments provide a solution permitting a steep stop-bandfarther from the pass band and taking up much less space than knownsolutions. This is possible, at least in part, due to the simplerconstruction presented herein. There is additionally provided a solutionpermitting balun operation and a change in impedance from one port toanother.

According to an embodiment, there is disclosed a bandpass filter withwide (e.g. 5% relative to center frequency) passband which is achievedusing lateral acoustic coupling between piezoelectric bulk acoustic wave(BAW) resonators. The device is preferably designed such that one bulkwave mode, e.g., first-order longitudinal thickness mode, is used toproduce the passband. With the correct design of the acoustic andelectric properties, a bandwidth comparable to or wider than that of thecurrent known acoustic filter components, i.e. surface or bulk acousticwave, SAW/BAW devices, is obtained at GHz frequencies, e.g. 1-5 GHz.

Some of the benefits of the embodiments are achieved by adding anacoustic resonator or resonators in series or in parallel with thefilters. Thus, by using the operating frequency of the resonator it ispossible to create a deep dip in the frequency response of the filterwhich both increasing the steepness of the edges of pass band andstop-band attenuation in the vicinity of the pass band.

There are several applications in which such possibilities are ofbenefit. The addition of a zero above and/or below the pass bandsteepens the edges of the pass band and improves stop-band attenuation.With the aid of parallel resonance in parallel resonators and seriesresonance in series resonators, pass-band attenuation can be reduced, orbandwidth increased.

It is then preferable to select the frequencies in such a way that aparallel resonator produces a zero below the pass band, and a seriesresonator produces a zero above the band. The stop-band attenuation inLBAW resonators increases as one moves farther from the pass band,whereas in ladder filters, for example, it decreases or remains the sameat a level determined by the capacitances of the resonators.

Furthermore, a LBAW filter can implement a balun functionality inside afilter, without a separate component. A balun filter with a steep passband and a simple manufacturing process is herein possible. Resonatorsalso act as matching elements for an LBAW filter. Compared to pureladder resonators, which have a steep band, embodiments of the presentinvention differ in the type of acoustic coupling and the filterconstruction. A LBAW plus resonators can take up less space, as fewerresonators are needed and stop-band attenuation far from the pass bandis better.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described with the aid of examplesand with reference to the accompanying drawings:

FIG. 1: (a) Schematic of a SAW device, (b) schematic of a SAWinterdigital transducer.

FIG. 2: (a) Solidly-mounted BAW resonator, (b) air-gap BAW resonator.

FIG. 3: Schematic of electrical connection of resonators in (a)T-section, (b) Pi-section. (c) 5-pole/2-section ladder filter. There isno acoustic connection between the resonators.

FIG. 4: (a) bulk mode propagation for two bulk modes (longitudinal andshear), (b) Schematic picture of dispersion curves of a BAW resonator.

FIG. 5: Schematic illustration of (a) a stacked crystal filter, (b)vertical SMR-type BAW CRF.

FIG. 6: Schematic of a two-electrode SMR LBAW structure.

FIG. 7: (a) Schematic illustration of the operation principle of a2-electrode LBAW. The two plate modes (even and odd) create a 2-polefilter response (b).

FIG. 8: Examples of electrical response of a vertically coupled CRF. Inthe example, minimum insertion loss is 1 dB and the 3-dB bandwidth in(a) is ˜80 MHz (4.3% at 1850 MHz center frequency).

FIG. 9: Microscope image of a 31-electrode LBAW designed and fabricatedat VTT. Electrode width W=2 μm, gap width G=2 μm, electrode length L=200μm.

FIG. 10: Dispersion curves calculated for the electrode region of theexample filter. TE1 cutoff frequency is ˜1850 MHz. TS2 k=0 frequency˜1780 MHz.

FIG. 11: Dispersion curves calculated for the outside region of theexample filter stack. TE1 cutoff frequency is 1935 MHz. TS2 k=0frequency is ˜1808 MHz.

FIG. 12: Measured electrical frequency response of a 31-electrode LBAWfilter designed and manufactured at VTT. Post-measurement (withsoftware) matching to 120Ω parallel to 5 nH at both ports was used.Center frequency is 1988 MHz, minimum insertion loss is 2.1 dB, andrelative 3-dB bandwidth is 97 MHz (4.9% of center frequency).

FIG. 13: Connection of resonators to an LBAW filter as a schematicdiagram and a circuit diagram.

FIG. 14: Simulated frequency response for a resonator.

FIG. 15: Simulated frequency response for a two-finger LBAW filter(black), a parallel resonator in the input (dark grey), parallel andseries resonators in the output (light grey).

FIG. 16: Simulated frequency response for a two-finger LBAW filter(black), a parallel resonator in the output (dark grey), parallel andseries resonators in the output (light grey).

FIG. 17: Simulated frequency response for a two-finger LBAW filter(black), parallel and series resonators in the input as well as aparallel resonator in the output (dark grey), parallel and seriesresonators in the input and in the output (light grey).

FIG. 18: Simulated frequency response for a nine-finger LBAW filter.

FIG. 19: Simulated frequency response for a nine-finger LBAW filterwhich has series and parallel resonators in the input (dark grey) and inthe output (light grey).

FIG. 20: Simulated frequency response for a nine-finger LBAW filter, thestop-band attenuation of which improves as the frequency moves away fromthe pass band.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The filter described herein is based on the SMR LBAW structure shown inFIG. 6. Electrodes in this example are interdigital (comb-like),although other geometries, e.g. circular, are possible as well.

The LBAW filters according to example embodiments are designed tooperate in the first-order thickness-extensional TE1 mode. This isbecause many piezoelectric thin film materials have electromechanicalcoupling stronger in the thickness direction, meaning that thelongitudinal vibration couples efficiently to the electrical excitationwhich is over the thickness of the piezoelectric layer.

Based on an exemplary demonstrator filter described below, an examplestructure for creating a wideband response is characterized by:

A BAW structure comprising an acoustic Bragg mirror 4, electrodes andpiezoelectric layer. While the piezoelectric material used in theexample is AlN, other 6 mm symmetry group piezoelectrics, such as ZnO,can be used as well. Furthermore, thin-film forms of other piezoelectricmaterials, such as known SAW materials like LiNbO3, LiTaO3, can be used.The BAW structure is further characterized in that it has Type 1dispersion.

The filter structure has interdigital electrode structure with twoports, in this case metal although other conductive materials can beused, with electrodes 5 and 6 deposited on the thin-film piezoelectriclayer. The electrode structure is designed such that the electrodes areconnected alternating to port 1 (input) 2 and port 2 (output) 3.

In an embodiment, the number of electrodes 5 and 6 is 31 in total. Theirwidth is 3 the gap between electrodes is 2 and the length of theelectrodes is 200 μm.

From the sake of clarity, the different regions of the layer stackhaving top metal will be referred to as the electrode region. The regionthat has no top metal and is outside of the electrodes is referred to asthe outside region.

According to an embodiment, the thin-film layer stack is designed asfollows. The mass loading by top the electrode, determined by the topelectrode thickness and material density, is such that the frequencydifference between the k_(∥)=0 frequency of the electrode region's TE1mode and the outside region's TS2 mode is small. More particularly, suchthat the k_(∥)=0 frequency of the outside region's TS2 mode is 95%-99%of the electrode region's TE1 cutoff frequency. In the present examplethe k_(∥)=0 frequency of the outside region's TS2 mode is 97.3%.

The frequency difference between the outside region's TS2 and TE1 modes'k_(∥)=0 frequencies is designed to be large, e.g. 5%-15% of theelectrode region's TE1 mode cutoff frequency. The frequency differencein the present example has been designed to be 6.7%.

According to certain embodiments of the present invention, the frequencydistance outside TS2 and TE1 is greater than, or equal to 98%, moreparticularly between 98% and 99.5%, more particularly still as shown inFIG. 4b as 98.9%. Similarly, the frequency distance expressed as thefrequency difference between TE1 and TS2 k=0 frequencies: (electrodete1−outside ts2)/outside ts2 should be small, for example on the orderof 1%. As an example, said frequency distance is preferably between 0.2%and 2.1%, more particularly between 0.5% and 1.8%, still moreparticularly between 0.8% and 1.5%, more particularly still as shown inFIG. 4B as 1.1%.

Another feature that has to be designed appropriately is the electrodetopology. The electrode topology should be designed such that the gapwidth G ensures good coupling at the even mode. For example, at leastfor the most common applications, the gap width should be designed to be20%-120%, preferably 25%-110% of the evanescent acoustic wave's decaylength, i.e. the length at which amplitude A=A₀*1/e of the originalamplitude A₀, in the gap at the desired even resonance mode.

Although an SMR type structure is used in the present example, thepresent invention is not limited to that type of structure. Otherstructures such as air-gap structures can be used as well, as long asthe acoustic properties are designed appropriately.

Additionally, other bulk vibration modes besides the TE1 can beutilized. However, the piezoelectric coupling of the driving electricfield to the used bulk acoustic mode should preferably be strong enoughso that low losses can be obtained.

Alternatively, gap width G can be determined normalized to thepiezolayer thickness d. For example, G can be designed to be 25%-200% ofd. In the present example G is designed as 102% of d.

Electrode width W is designed such that multiple half-wavelengths cannotfit within the electrode width. For example, W is designed to be smallerthan the lateral acoustic wave's wavelength λ_(odd) at the desired oddresonance mode.

The number of electrodes N, electrode width W and gap width G aredesigned such that the desired wavelength of the lateral acoustic waveat the even mode resonance frequency is achieved. That is,N*W+N*G=λ_(even)/2, where λ_(even) is the wavelength of the lateralacoustic wave at the even mode resonance frequency. For present example:N=31, W=3 and G=2. Additionally, the total width of the structureN*W+N*G is such that the highest-order mode trapped in the structure isthe desired odd mode resonance.

The electrode width W is designed such that the wavelength of thelateral acoustic wave at the desired odd mode resonance frequency,λ_(odd), is obtained. For example: W=25%-50% of λ_(odd).

The tables 1-3 below define acceptable thickness ranges for thin-filmlayers according to typical embodiments of the present invention. Thepiezo layer thickness d (minimum and maximum value) is first determinedwith respect to the acoustic wavelength in the piezo material (λ) at theoperation frequency f Thicknesses (min and max) of top electrode (tope),bottom electrode (bote) and the topmost mirror layer (M1) are thendefined with respect to the piezo layer thickness. Tope, bote and M1 arelabeled in the general schematic of FIG. 6(a). The mirror layersunderlying the topmost mirror layer M1 are designed such that therequired wave reflectivity is obtained. A starting point can be, forexample, a thickness that equal one quarter of the acoustic wavelengthin the material.

Device operation is dependent on the combination of these layerthicknesses. Therefore the thicknesses are not independent. More thanone layer may contribute to one acoustic or dispersion property, and onelayer may also contribute to several properties.

TABLE 1 Wide range layer thickness percentage Typical Min, % Max, % Min,% Max, % Layer material Symbol of λ of λ of d of d Piezo AlN d 23 50 TopAl tope 0.8 7.5 3.4 22 electrode Bottom Mo bote 1.3 20 5.7 40 electrodeTop SiO2 M1 9.7 35 42 70 mirror

TABLE 2 Medium range layer thickness percentage Typical Min, % Max, %Min, % Max, % Layer material Symbol of λ of λ of d of d Piezo AlN d 2741 Top Al tope 1.3 4.2 4.9 10.3 electrode Bottom Mo bote 3.1 1189 11.429 electrode Top SiO2 M1 12.7 25.8 47 63 mirror

TABLE 3 Tight range layer thickness percentage Typical Min, % Max, %Min, % Max, % Layer material Symbol of λ of λ of d of d Piezo AlN d 3037 Top Al tope 1.7 3.5 5.7 9.5 electrode Bottom Mo bote 4.5 7.0 15 19electrode Top SiO2 M1 15 21.5 50 58 mirror

Table 3 is representative of the present example. When the piezo layerthickness is chosen to be 1960 nm (35% of λ) as in the present example,the different ranges provide the following layer thicknesses (in nm) intables 4-6:

TABLE 4 Wide range layer thickness values Typ. Thickness Thickness Layermaterial min (nm) max (nm) piezo AlN 1960 tope Al 67 294 bote Mo 112 784M1 SiO2 823 1372

TABLE 5 Medium range layer thickness values Typ. Thickness ThicknessLayer material min (nm) max (nm) piezo AlN 1960 tope Al 96 202 bote Mo223 568 M1 SiO2 921 1235

TABLE 6 Tight range layer thickness values Typ. Thickness ThicknessLayer material min (nm) max (nm) piezo AlN 1960 tope Al 112 186 bote Mo294 372 M1 SiO2 980 1137

Once the design is chosen, it is determined if matching to the systemimpedance level is achieved. The design, i.e. N, W, and electrodelength, is then modified when necessary, such that matching to thesystem impedance level is achieved while retaining a desired loss levelwithin the passband.

1.

Steepness of passband edges can be improved by adding, in paralleland/or in series, resonators before and/or after the filter.

Example Filter with Measurement Results

One exemplary example is described herein along with the correspondingmeasurement results from the example filter. The structure of the filterwill now be described below.

The filter of the present example consists of a thin-film stack. Thenominal thin-film layer thicknesses of the acoustic stack are set forthin Table 7, from bottom to top. The substrate of the filter in theexample is silicon. The electrode structure used is interdigital andsimilar to that shown in FIG. 1(b). A microscope image of the presentexample is also presented in FIG. 9. The total number of electrodes in 5and 6 is N=31, the electrode width W=3 μm, the gap width G=2 μm, theelectrode length L=200 μm and the probe pad size is 150 μm×150 μm.

Although the electrode and gap width W and G are constant in FIG. 1(b)as well as in the component introduced below in FIG. 9, the structureneed not be limited to constant W/G and/or constant period.

TABLE 7 Nominal layer thicknesses of the stack in measured devices.Layer SiO2 W SiO2 W SiO2 Mo AlN Al Thickness (nm) 790 505 620 510 1030300 1960 150

Calculated dispersions for the electrode and outside regions are shownin FIGS. 10 and 11. S-parameter measurements were done on-wafer with avector network analyzed (HP8720D). Measurement was done at 50Ω systemimpedances. As-measured results were matched (in Matlab) after themeasurement. Insertion loss IL=20 log 10(|S21|) is plotted in FIG. 12.

Within the frequency range of 1-5 GHz, the bandwidth at 2 GHz was foundto be 5%. This is compared to 4% at 2 GHz of the closest prior artversion. Therefore, an increase of 25% bandwidth is realized in thepresent example. One of ordinary skill in the art will immediatelyrecognize the advantage such an increase in bandwidth at 2 GHzrepresents to the state of the art.

Furthermore, when looking at the required pattern resolution at 2 GHz,the present example critical dimension was greater than 1 μm. Comparedto a standard SAW coupled resonator filter at 2 GHz which has a requiredcritical dimension of around 0.5 μm, there is seen an over 100% increasein resolution requirement using this embodiment.

A second example is presented herein with ZnO as the piezoelectricmaterial. The thin-film stack of this example filter is shown in Table8.

TABLE 8 Layer thicknesses in the second example device from bottom totop. Layer SiO2 W SiO2 W SiO2 Mo ZnO Al Thickness 746 654 746 654 746300 d = 1100 200 (nm) % of d 67 27 d = 32% 18 of λ

The simulated matched passband width obtained with the example stack ofTable 8, number of electrodes N=31, electrode width W=6 μm and gap withG=3 μm is 90 MHz at 1933 MHz center frequency (4.7%).

Another example is obtained by replacing the mirror stack of theprevious example (Table 8) with an air gap. That is, all layers belowbottom electrode (Mo) are removed. The simulated matched bandwidthobtained with N=11, W=6 μm and G=3 μm is 114 MHz at 1933 MHz (5.9%).

Furthermore, ‘laterally coupled solidly mounted BAW Resonators at 1.9GHz’. Julkaisussa proceedings of international ultrasonics symposium2009. IEEE, 2009 s. 847-850 by Meltaus et al., is herein incorporated byreference in its entirety.

Modifying Steepness of Passband Edges

A laterally acoustically coupled filter (LBAW) has several advantagesover ladder filters consisting of separate resonators. These advantagesinclude smaller size, better stop-band attenuation, the possibility toimplement a change from a balanced signal to an unbalanced one (balun)without a separate component, as well to modify the impedance levelbetween the ports. However, LBAW filters have by nature gentler passband edges than, for example, ladder filters, which limits theirpossible applications.

As can be seen from FIGS. 13-20, series and/or parallel BAW resonatorsare used in the input and output to position zeros, i.e. attenuationpeaks in the frequency response of the LBAW filter. The frequencies ofthe attenuation peaks can be determined by altering the mass load of theresonator. For example, altering can be done by etching an AlN layer,growing additional material on top of the resonator or other suitablemethod known in the art.

It is possible to use various combinations of series and parallelresonators to shape the end response. Furthermore, the device'simpedance, i.e. matching, can be adjusted through the size of theresonator.

FIGS. 13-20 show how the frequency response of an LBAW filter can beshaped with the aid of simple series and parallel resonators. With theaid of zeros set on both sides of the pass band, the edges of the passband can be steepened while still retaining a good band shape. Theresonators also improve stop-band attenuation near to the pass band andact as transmatch circuits.

FIG. 13 shows a schematic diagram of a LBAW filter with series andparallel resonators. The most suitable filter combination can beselected for each construction. The filter has a piezoelectric layer 111supporting, a parallel resonator 112 and a series resonator 113 at theinput of a LBAW filter 114, as well as a series resonator 115 and aparallel resonator 116 at the output of the LBAW filter 114.

Acoustic series 113, 115 and/or parallel resonators 112, 116 can beconnected to the input and output. Various combinations of theresonators described are possible. In the parallel resonators 112, 116the lower electrode is grounded. In the series resonators 113, 115 thesignal goes to the lower electrode.

The following simulations shown in FIGS. 14-20 are based on an acousticthin-film stack, the materials and film thicknesses of which (from thesubstrate upwards) are listed in Table 9 below.

TABLE 9 Thin-film pack used in the simulations Material Si SiO2 W SiO2 WSiO2 Ti Mo AlN TiW Al Thickness nm Substrate 786 505 621 507 1029 25 2961960 17 106

The frequency of the series 113, 115 and parallel resonators 112, 116can be altered, in such a way that the resonator's series and parallelresonances will occur at the desired frequencies. The frequency can beadjusted by altering the thickness of one or more thin films. Thethickness of the piezoelectric film has been altered by thinning ormaterial can be grown on top of the resonators to form a mass load, inwhich case the resonance frequency will decrease.

FIG. 14 shows the frequency responses of three BAW resonators, simulatedusing 1D models. The black curve is the resonance response 121 producedby the stack of Table 1. The light-grey curve 122 corresponds to afrequency-shifted resonator, with an AlN layer that has been thinned to1900 nm. This resonator is used as the parallel resonator in thesimulations. The dark-grey curve 123 corresponds to a frequency-shiftedresonator that has been correspondingly thinned to 1810 nm. Thisresonator is used as the series resonator in the simulations.

The simulated frequency response in FIG. 14 is for a resonator sized 100μm×100 μm using the stack (black) of Table 1. The frequency of theseries resonance of the unmodified resonator 121 a is 1815 MHz and thefrequency of the parallel resonance 121 b is 1860 MHz. The light-greyand dark-grey curves are the simulated responses for thefrequency-shifted resonators. The series resonance 122 a of the parallelresonator has been shifted to the frequency 1850 MHz, which is below thepass band, so that the parallel resonance 122 b is at the frequency 1900MHz, i.e. it is on the pass band. The series resonance 123 a of theseries resonator is 1905 MHz (on the pass band) and the parallelresonance 123 b is 1960 MHz (above the pass band). The filter's responsereceives powerful attenuations at the series resonance frequency of theparallel resonator and at the parallel resonance frequency of the seriesresonator.

Unwanted lateral responses will arise in a real two-dimensionalresonator, which 1D simulation does not take into account. To ensure thebest response, it would be good to use some structure, which attenuatesthe lateral resonance shapes, in the resonators. The followingsimulations present the responses of two different LBAW constructionsand the effects of the resonators on the electrical frequency response.

FIG. 15 pertains to a two-finger LBAW filter. In the figure, thesimulated frequency response of a two-finger LBAW filter based on thestack of Table 1 is shown in black, 131. The response is matched with animpedance of 500 Ohm. On the right-hand side of the pass band is a zeropoint produced by the acoustic structure, but the lower edge of the bandis very gentle.

If a parallel resonator 112 is added to the input of the construction,i.e. response 122 of FIG. 14, a strong attenuation peak, which appearsas the dark-grey response 132 in FIG. 15, arises at the series-resonancefrequency (1850 MHz) below the pass band. Therefore, stop-bandattenuation improves by more than 10 dB below the pass band and by about7 dB above it. As can be seen though, at the same time the bandwidthdecreases somewhat.

When a series resonator 113 is also added to the input, i.e. theresponse 123 of FIG. 14, an attenuation peak is obtained at its parallelresonance frequency (1960 MHz). The steepness of the upper edge of thepass band improves and the stop-band attenuation improves by some dB,compared to only a parallel resonator 112.

FIG. 15 uses a black curve to show the resonance response 131 of atwo-finger LBAW filter, a dark-grey curve to show the resonance response132 if there is a parallel resonator in the input of the filter 131, anda light-grey curve to show the resonance response 133 if there areparallel and series resonators in the input of the filter 131.

FIG. 16 shows the same resonator combinations in the output port. FIG.16 uses a black curve to show the resonance response 141 of a two-fingerLBAW filter, a dark-grey curve to show the resonance response 142 ifthere is a parallel resonator in the output of the filter 141, and alight-grey curve to show the resonance response 143 if there areparallel and series resonators in the output of the filter 141. Aparallel resonator will have nearly the same effect as in the inputport, but a series resonator will create a peak in the response.

FIG. 17 shows other possibilities. FIG. 17 uses a black curve to showthe resonance response 151 of a two-finger LBAW filter, a dark-greycurve to show the resonance response 152 if there are parallel andseries resonators in the input and a parallel resonator in the output,and a light-grey curve to show the resonance response 153 if there areparallel and series resonators in both the input and the output.

The best result is achieved if both a series and a parallel resonatorare added to the input and a parallel resonator is added to the output.With this configuration band-edge steepness improves considerably.Stop-band attenuation improves below the band by nearly 30 dB and aboveit by nearly 15 dB. At the same time, the width of the pass banddecreases to some extent. In accordance with the present example thedecrease is as follows; absolute 5 dB pass band: 55 MHz (2.9%)->42 MHz(2.2%) where the percentage is compared to a frequency of 1900 MHz.

The examples according to FIGS. 18-20 deal with a nine-finger LBAWfilter. As shown, by increasing the number of fingers the filter'sbandwidth can be increased.

FIG. 18 uses black to show the simulated frequency response 161 of a9-finger LBAW filter based on the stack of Table 1 with the responsematched by an impedance of 100Ω. The bandwidth (abs. 5 dB) is 78 MHz(4.1%), but both edges of the pass band are very gentle.

By adding a parallel resonator, such as with frequency response 122 tothe input, the dark-grey frequency-response curve 162 is obtained, inwhich there is an attenuation peak at a frequency of 1850 MHz. A seriesresonator with the example frequency response 123 in the input gives thelight-grey curve 163, in which there is an attenuation peak at afrequency of 1960 MHz. Both resonators in the input create the dark-greycurve 164, in which there are peaks at both frequencies. As can be seen,the upper edge of the pass band is both wider and better shaped, i.e.there is smaller insertion attenuation. The stop-band attenuationimproves below the pass band by about 3 dB and above it by about 2 dB.Unlike a ladder filter, the stop-band attenuation increases as one movesaway from the pass band frequency.

In FIG. 19, series and parallel resonators in the input and output arecompared. FIG. 19 uses black to show the simulated frequency response171 of a 9-finger LBAW filter based on the stack of Table 1. Bothresonators in the input create the dark-grey curve 172, in which thereare peaks at the frequencies 1850 MHz and 1960 MHz of the previousexample as well. On the other hand, the adding of both resonators to theoutput instead of the input weakens the stop-band attenuation, accordingto the light-grey curve 173.

FIG. 20 uses black to show the simulated frequency response 181 of a9-finger LBAW filter based on the stack of Table 1. Both resonators inthe input create the dark-grey curve 182, in which there are peaks atthe frequencies 1850 MHz and 1960 MHz of the previous example as well.In addition to the above, the addition of a parallel resonator to theoutput improves the response slightly, according to the light-grey curve183. The absolute 5-dB bandwidth of the filter achieved is 77 MHz (4%).The stop-band attenuation improves as one moves away in frequency fromthe pass band.

As is seen from FIGS. 14-20, the use of various combinations ofresonators allows for the tailoring of LBAW frequency responses. Whileseveral combinations of resonators has been shown herein, based on thedisclosed description, one of ordinary skill in the art will recognizefurther combinations, and sub-combinations, which achieve the same orsimilar goals as the presented combinations without departing from thescope of the invention.

The present invention is not limited to the exemplary examples describedherein. The exemplary examples and embodiments merely show some of theadvantageous effects of a filter designed according to the presentinvention. However, one of ordinary skill in the art will recognizeobvious variations and combinations of elements described herein whichdo not part from the scope of the present invention.

Generally, to obtain a filter according to the present embodiments withsome or all of the desired, advantageous effects, the following shouldnormally be designed or accounted for in the filter design; correctacoustic properties i.e. dispersion, TE1 mode is trapped in thestructure (outside region preferably has evanescent wave at operationfrequency range), maximum frequency difference between even and oddresonance modes, long lateral wavelength at the even mode (long decaylength outside the electrodes), high enough Q value for low losses,appropriate electrode design, odd mode is trapped, insuring fabricationtolerances are not critical e.g. not too narrow gaps, matching electrodelength and number of electrodes (not too much resistance), choosingsmall enough component size, insuring no intermediate lateral modes thatwould produce notches in the passband.

When taking into account some or all of the factors mentioned above, itis possible to produce a filter with extremely wide passband and asimple fabrication process. Wide obtainable band gives freedom to designusing wider electrodes and gaps. More electrodes eases the matching to50 Ohms. The need to use matching inductances or other matching elementsis smaller and the requirement to gap width is relaxed allowing foreasier fabrication. Better performance (lower losses etc) can beachieved when there is enough bandwidth to sacrifice a portion of thebandwidth.

Other materials besides 6 mm symmetry group can be utilized such asthin-film forms of piezoelectric SAW materials e.g. LiNbO3 and LiTaO3.Additionally, operation frequency does not principally depend onelectrode dimensions, but on film thicknesses. Therefore high frequencyoperation is possible and lithography is not a limiting factor.

As described above, the thin-film stack and electrode geometry can bedesigned such that the dispersion properties are beneficial forwide-band operation. The long wavelength and good acoustic coupling ateven resonance mode can be ensured by bringing the electrode region'sTE1 curve (if TE1 is the operating mode) close to gap region's TS2curve. This makes possible long decay length in the gaps at even mode.At the same time, the frequency range within which the modes are trappedin the electrode structure becomes wider. Furthermore, the large numberof electrodes (N>10) can be used to get long wavelength (low frequency)at even mode and short wavelength (high frequency) at even mode.

As the dispersion and wave behaviour in the complex structure describedherein (multi-layer, lateral topology) is affected by several factors atonce (layer materials and thicknesses, lateral geometry), some of whichhave opposite effects, it is extremely unlikely that optimal designcould be found by trial and error. As such, the designer must have aclear idea of the way the properties need to be modified, as disclosedherein, in order to be able to create a useful filter according to thepresent invention.

The present invention is not limited to the exemplary embodiments andexamples described herein. They are meant only to help describe thepresent invention. Further examples are described in U.S. Provisionalapplication 61/392,955 for which the present application claims priorityfrom and which is herein incorporated by reference in its entirety.Numerous variations in manufacturing processes, number of iterations,types of novel products and more will be recognizable to one of ordinaryskill in the art without departing from the scope of the presentinvention.

1. An acoustically coupled thin-film Bulk Acoustic Wave (BAW) filter,comprising: a piezoelectric layer, an input-port on the piezoelectriclayer changing electrical signal into an acoustic wave, and anoutput-port on the piezoelectric layer changing acoustic signal intoelectrical signal, wherein the ports include electrodes positioned suchthat acoustic coupling is achieved, the filter is capable of operatingin the first order thickness-extensional TE1 mode, and wherein the massloading by the top electrode (plateback) is such that the frequencydifference between the k=0 frequency of the electrode region's TE1 modeand the outside region's TS2 mode is relatively small.
 2. The filter inaccordance with claim 1, wherein the k=0 frequency of the outsideregion's TS2 mode is between 93% and 99.9%, in particular between 95%and 99%, more particularly between 97% and 98% of the electrode region'sTE1 cutoff frequency.
 3. The filter in accordance with claim 2, whereinthe k=0 frequency of the outside region's TS2 mode is between 98% and99.9% of the electrode region's TE1 cutoff frequency.
 4. The filter inaccordance with claim 1, wherein the electrodes are positioned such thatacoustic vibration in the lateral direction from one electrode to theother acoustically couples the electrodes.
 5. The filter in accordancewith claim 1, wherein the filter structure that has interdigitalelectrode structure with two ports, such that the electrodes areconnected alternatingly to the input port and the output port.
 6. Thefilter in accordance with claim 1, wherein the electrode topology issuch that gap width G ensures good coupling at the even mode.
 7. Thefilter in accordance with claim 6, wherein the gap width is between 20%and 120%, in particular between 25% and 110%, of the evanescent acousticwave's decay length in the gap at the desired even resonance mode, wherethe wave's decay length is expressed as the length at which amplitudeA=A0*1/e of the original amplitude A0.
 8. The filter in accordance withclaim 1, wherein the electrode width W is such that more than onehalf-wavelength of the lateral acoustic wave's wavelength cannot fitwithin the electrode width.
 9. The filter in accordance with claim 1,wherein the electrode width W is smaller than the lateral acousticwave's wavelength λodd at the desired odd resonance mode.
 10. The filterin accordance with claim 1, wherein the number of electrodes N,electrode width W and gap width G are designed such that the desiredwavelength of the lateral acoustic wave at the even mode resonancefrequency is achieved.
 11. The filter in accordance with claim 10,wherein N*W+N*G=λeven/2, where λeven is the wavelength of the lateralacoustic wave at the even mode resonance frequency, and that thehighest-order mode trapped in the structure is the desired odd moderesonance.
 12. The filter in accordance with claim 1, wherein theelectrode width W is such that the wavelength of the lateral acousticwave at the desired odd mode resonance frequency, λodd, is obtained. 13.The filter in accordance with claim 12, wherein W is between 25% and 50%of λ_(odd).
 14. The filter in accordance with claim 1, wherein matchingto the system impedance level is achieved, while retaining a desiredloss level within the passband by the combination of N, W, and electrodelength L.
 15. The filter in accordance with claim 1, wherein thepiezoelectric layer is formed on an acoustic Bragg structure formed as athin film stack or on an air-gap structure.
 16. The filter in accordancewith claim 1, further comprising there being one or more resonatorsadded to the filter in parallel and/or series.
 17. The filter inaccordance with claim 16, wherein one or more of the resonators arecoupled before and/or after the filter.
 18. (canceled)